Update PR template with instructions about JIRA

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,9 @@
 Dear Airflow Maintainers,
 
-Please accept the following PR that
-* Addresses the following issues (links to issues addressed by this PR) :
+Please accept this PR that addresses the following issues:
+- *(replace with a link to AIRFLOW-X)*
 
-Reminder to contributors:
-* You must add an Apache License header to all new files
-* Please squash your commits when possible and follow the [7 rules of good Git commits](http://chris.beams.io/posts/git-commit/#seven-rules)
+Reminders for contributors:
+* Your PR's title must reference an issue on [Airflow's JIRA](https://issues.apache.org/jira/browse/AIRFLOW/). For example, a PR called "[AIRFLOW-1] My Amazing PR" would close JIRA issue #1. Please open a new issue if required!
+* You must add an [Apache License header](http://www.apache.org/legal/src-headers.html) to all new files.
+* Please squash your commits when possible and follow the [7 rules of good Git commits](http://chris.beams.io/posts/git-commit/#seven-rules).